Generally, in an image forming apparatus, the image density of a toner image may fluctuate due to disturbances such as changes in the surrounding environment such as temperature and humidity, and temporal changes of the apparatus. Accordingly, in an image forming apparatus, in general, gradation correction is performed to stabilize the image density.
For example, in an image forming apparatus, a gradation pattern image consisting of a plurality of gradation levels is formed on a photoconductor as a test toner image, and gradation correction is performed based on the densities of this gradation pattern image. This gradation correction is performed to correct image gradation characteristics when turning on power, when returning from sleep, when the print count has reached a predetermined number, or when the external environment has significantly changed.
To be specific, in the image forming apparatus, a gradation pattern image is formed on an intermediate transfer belt at the time of gradation correction, and the density of each gradation level is detected by a density sensor. Then, a desired image density is obtained by adjusting a developing bias such that the detected densities match their respective predetermined target densities (that is, by performing gradation correction). In reality, this gradation correction is performed by correcting an input gradation of image data using a gradation conversion table of a control section such that each gradation level has a predetermined density based on the density detection value obtained by a density sensor.
Here, if the detection accuracy of the gradation pattern image is poor, the accuracy of gradation correction will deteriorate accordingly; therefore, up to now, various contrivances have been made to detect the density of gradation pattern image at a high accuracy. Patent Literatures 1 and 2 disclose a technique to use a plurality of gradation pattern images. Moreover, Patent Literatures 3 to 8 disclose a technique in which to cancel density unevenness (cyclic density unevenness) that occurs at the cycle of a developing roller, an intermediate transfer belt, or a photoconductor, a plurality of gradation patterns are positioned at an interval of ½ or M/N (where M and N are coprime natural numbers) of the above-described cycles.